Xerographic plate



g- 25, 1959 R. M. SCHAFFERT ETAL 2,901,349

XEROGRAPHIC PLATE Filed May 23, 1957 I I4 II/III/I/IIII/II/I/I/lINVENTOR. R. M. fferf BY J. F.

E ATTORNEY United States Patent O XEROGRAPHIC PLATE Roland M. Schaifert,Vestal, N.Y., and James F. Hansen, Billiards, Ohio, assignors, by mesneassignments, to Haloid Xerox Inc., a corporation of New York ApplicationMay 23, 1957, Serial No. ssnoss '4 Claims. 01. 96-1 This inventionrelates in general to the art of xerography, and, in particular, to asensitive plate therefor. Morespccifically, the invention relates to anew xerographic member comprising a conductive backing having on atleast one surface thereof a coating of arsenic trisulfide, which in turnis covered by a coating of a photoconductive insulating material, whichmember is known as a xerographic plate. I

In the art of xerography it is usual to form an electrostatic latentimage on a member or plate which comprises a conductive backing such as,for example, a metallic SUI-1 face, and having a photoconductiveinsulating surface thereon. It has previously been found that a suitableplatefor this purpose is a metallic member having a layer of vitreousselenium. Such a plate is characterized by being capable of receiving asatisfactory electrostatic charge and selectively dissipating such acharge when ex-. posed to a light pattern. While such a plate has becomethe commercial standard in the xerographic art, it is subject to certainlimitations. Thus, due to an extremely rapid loss of potential whenstored in the dark (termed dark decay.) for a negative charge the use ofselenium plates is largely restricted to positive charging. Furthermore,when selenium plates are used with extremely penetrating radiation, suchas X-rays, the dark decay increases enormously after exposure to thepenetrating radiation. Thus, before the plate can be resensitized andused in another cycle of operation special measures must be taken torestore; the plate to its insulating condition wherein an electrostaticcharge will be satisfactorily retained in the dark. In practice, thisrequires resting the plate for appreciable periods of time.

Now, in accordance with the present invention, it has been found that animprovedxerographic plate can be prepared by depositing a layer ofarsenic trisulfide between the conductive backing and thephotoconductiye insulating layer. The plate, as thus modified, may beused equally with .both positive and negative charging without loss inspectral sensitivity and exhibits an astonishing. degree of freedom fromfatigue when used with penetrating radiation.

The new and improved plates of the present invention can be prepared bya variety of methods. For example, the separate layer of arsenictrisulfide and photoconduc tive insulating material may be applied byevaporation onto the plate under high vacuum, by spraying on the desiredsurface in molten form, by flame deposition (.de

ice

composing a volatile organic compounds when in contact with the surfaceon which the coating is made), etc. If desired a transparent insulatingcoating as of a vinyl resin, a cellulose ester or ether, a siliconeresin, silicon monoxide, etc. may be coated on top of the xerographicplate to protect the surface thereof from abrasion and mechanicaldamage.

While selenium is the preferred photoconductive insulating material,other known photoconductive ins'ulat ing materials as anthracene, sulfurand various alloys of selenium such as mixtures of selenium andtellurium, selenium and arsenic, selenium and gallium, etc. may be usedas the photoconductive insulating layer. In addition, thephotoconductive insulating layer may, if desired, be itself formed indiscrete layers each layer containing a different type ofphotoconductive insulating material. One such combination would be alayer of vitreous selenium immediately in contact with the arsenictrifluoride while on top of the vitreous selenium would be placed alayer of a mixture of about 20% tellurium and 80% selenium by Weight.Other photoconductive insulating materials and combinations thereofknown to those skilled in the art may be used in preparing the novelplates of the instant invention. r

The drawing is a section of a xerographic plate prepared according tothe invention. As shown, a xerographic plate 10 according to the instantinvention, comprises a conductive backing 11 having coated thereon athin relatively uniform layer 12 of arsenic trisulfide which in turn iscovered by a layer 13 of a photoconductive insulating material. Ifdesired, the photoconductor 13 may be coveredby a protective coating 14.i

The general scope and nature of the invention having been set forth thefollowing examples are given as typical illustrations of methods bywhich the desired plates may be prepared.

EXAMPLE 1 A brass plate was polished with Glass Wax (a trade name of theGold Seal Company, Bismarck, North Dakota, for a composition comprisingabout 75% water,

15% naphtha, 7.5% abrasive, and the balance ammonia,

emulsifier, and coloring agent), rinsed in isopropyl alcohol and thendegreased in hot isopropyl alcohol vapor. A mask was placed over half ofthe plate. The plate was then attached to a platen about four inchesabove a molybdenum boat. Yellow orpiment-arsenic trisulfide oba platewas placed in the dark and charged negatively-by tained from the Colemanand Bell Company, Norwood, Ohio, was placed in the boat and a bell jarplaced over the apparatus. The system Was then evacuated to a pressureof approximately 0.5 micron of mercury and the corona emission. Theamount of charge on the plate was then measured with an electrometer.The plate was kept in the dark for some time during which severalmeasurements were made of the charge on the plate to determine the darkdecay taking place.

In the case of the selenium only portion of the plate, the dark decaywas so rapid that plate potential had fallen off to 60 volts before itcould be measured, while in the case of the portion of the plate havingan interlayer of arsenic trisulfide the initial reading was 500 volts.The dark decay half-time from these initial potentials measured inseconds were 20 for the selenium only portion of the plate and 817 forthe portion having the arsenic trisulfide interlayer.

The. experiment Was then repeated using positive corona emission tocharge the plate. With positive charging both the selenium only and thearsenic trisulfide interlayer portions of the plate accepted an initialpotential of 500 volts. However, the dark decay half-time was 650seconds for the selenium only portion and 2,220 seconds for the portionwith the arsenic trisulfide interlayer.

The sensitivity of the two portions of the plate to light of variouswave lengths was then determined for both positive and negativesensitization. The light intensity for all wave lengths was 0.03microwatt per centimeter. The sensitivity was computed by the standardmethod of using the reciprocal of the time for the potential to drop toone-half its initial value (from V to V /2) under exposure to this lightintensity. The formula is where S equals sensitivity, T is the time inseconds for the potential drop to one-half its initial value (from V toV /Z, V is the initial potential and I is the light intensity. Usingthese figures with a monocromatic light source neither portion of theplate displayed a measurable sensitivity for either positive or negativecharging for wave lengths of either 600 or 700 millimicrons.

For negative charging the dark ldecay was too rapid to permitmeasurement of sensitivity at any wave length for the portions of theplate having only selenium. In the portion of the plate having anarsenic trisulfide interlayer it was found that the plate had asensitivity of 2.1 at a wave length of 500 millirnicrons and of 7.8 at400 milli-. microns.

For positive charging the portion of the plate having only selenium hada sensitivity of 3.9 at 500 millimicrons and 13.4 at 400 millimicrons.In contrast, the portion having an arsenic trisulfideinterlayer hadsensitivities of 4.2 at 500 millimicrons and 18.7 at 400 millimicrons.

EXAMPLE 2 A plate was prepared as in Example 1 except that the arsenictrisulfide interlayer was less than one micron thick while the seleniumremained at 35 microns.

In this plate for positive charging charge acceptance for both portionsof the plate was 500 volts. However, the dark decay half-time was only460 seconds for the selenium only portion but 29,000 seconds for thearsenic trisulfide interlayer portion. The seleniumv only portion of theplate had a sensitivity of 4.7 at 500 millimicrons and 13.4 at 400millimicrons while the portion having the arsenic trisulfide interlayerhad sensitivities of 2.7 at 500 millimicrons and 12.6 at 400millimicrons.

For negative charging it was again almost impossible to, measure theselenium only portion of the plate. The initial potential was only 60volts and the dark decay half-time was only 32 seconds. In contrast, theportion of the plate having the arsenic trisulfide interlayer had acharge acceptance for negative charging of 4501 volts, a' dark decayhalf-time of 500 seconds, a sensitivity'of 3.8'

atz500 millimicrcns and at 400 millimicrons.

4 EXAMPLE 3 A plate was prepared as described in Example 1 except thatin place of the Glass Wax treatment the plate was rinsed successively inbenzene, isopropyl alcohol, and acetone prior to the degreasing in hotisopropyl alcohol vapor. The selenium coating was about microns thickand the arsenic trisulfide interlayer was about 0.2 micron thick. Thisplate was used to test X-ray fatigue and sensitivity.

An objective measure of fatigue is obtained by dividing the difierencein voltage on the plate before and after exposure to radiation (taken.in each case three minutes after charging) by the voltage on the platebefore exposure (three minutes after charging), expressing the ratio asa percentage. At no time during the fatigue measurements were the platesexposed to light and the plates were allowed to rest in the dark aminimum of seven hours between successive exposures to X-rays. Afterbeing allowed to rest in the dark for the prescribed period the plateswere charged and the dark decay was observed for three minutes. Theplates were then exposed to X-rays and after waiting for from a halfminute to thirty minutes were again charged and the dark decay againobserved for three minutes. In each case the 60 ma.-second exposure toX-rays was suflicient to decay the plates to zero potential. The X-raytube was placed 36 inches above the plate and exposure was through a &inch Bakelite window at 100 kilovolts.

In the case of the selenium only portion of the plate, the fatigue was75% when measured one-half minute after exposure and was still 23.2%when measured 30 minutes after exposure. In comparison, the portion ofthe plate having an interlayer of arsenic trisulfide had a fatigue ofonly 15.4% when measured one-half minute after exposure and this hadfallen to 0% fatigue at 10 minutes after exposure.

In this series of tests the plates were charged by corona emission to aninitial potential of 1,000 volts as is usual in xeroradiography.

The fatigue tests were then repeated using negative charges to sensitizethe xerographic plate. When the recharging occurred one-half minuteafter exposure, the fatigue on the selenium only plate was 76% fallingto 27% when the charging occurred 30 minutes after exposure. On theportion of the plate having the arsenic trisulfide interlayer, when thecharging occurred one-half minute after exposure, fatigue once again was15.5% falling to 2.5% when charging occurred five minutes afterexposure.

The X-ray sensitivities of the two portions of the plate were alsodetermined in terms of the percentage rate of decay of original charge,i.e., percentage divided by secends with the following results: Aselenium plate positively charged had an X-ray sensitivity of 7.6 whilethe plate with the arsenic trisulfide interlayer had a sensitivity of10.2 when positively charged and 17.0 when negatively charged.

EXAMPLES 4 THROUGH 6 Three plates were prepared, one having an aluminumbacking, one aluminum covered with a coating of aluminum oxideapproximately 40 angstroms thick and one having a chromium backing. Thealuminum backing was cleaned by successive rinses in benzene, isopropylalcohol, acetone and hot isopropyl alcohol vapor; In each case one-halfof the plate was coated with arsenic trisulfide by vacuum evaporation asdescribed in Example 1, the layer being about 0.5 micron thick, and thenthe entire plate was covered with selenium-again by vacuum evaporation.The selenium thicknesses were, respectively, microns, 135 microns andmicrons. Plates were then tested for initial charge acceptance, darkdecay rate measured in volts per minute and sensitivity to both 400millimicron and 500 millimicron wavelength radiation and fatigue. Thefatigue test was the same as in Example 3 except that the radiation usedwas 400 millimicrons wavelength rather than X-ray radiation. The plateof Example 6 was also tested using negative charging. Results of thesetests are set forth in the following table:

. 6 of a cylinder, flexible sheet or other member having a surfacesuitable for the xerographic process.

. The selenium used in the preparation of xerographic plates should befree of impurities such as copper, iron,

Table 1 Dark- Sensitivit Fati e Charging tial Decay y gu Example FilmComposition Polarity Potential, rate,

volts volts/min. 400m 500111;; 0.5 5.0

min.

4 Selenium Se+As S Interface g 3 5 do 300 7 19. 5 9. 8 1s. 5 1. 7 400 429 8.7 2.9 0 6 do 420 14 6.5 6.7

as 1.; a 0 i- 310 '2 20 512 IIIIIIIIIIIIIIII EXAMPLES 7 THROUGH 9 Threeplates were prepared, two having brass backings and one aluminum coveredwith a coating of aluminum oxide approximately angstroms thick. In eachcase one-half of the plate was coated with arsenic trisulfide by vacuumevaporation as described in Example 1, the layer being about 0.5 micronthick, and the entire plate was then covered with vitreous seleniumagain by vacuum evaporation as described in Example 1. The seleniumthicknesses were, respectively, 115 microns, 143 microns and 178microns. The plates were then tested for initial charge acceptance, darkdecay rate measured in volts per minute and sensitivity to both 400millimicron and 500 millimicron wavelength radiation. Results of thesetests, for both positive and negative charging, are set forth in thefollowing table:

lead, and bismuth, which appear to adversely aifect its ability to holdelectrostatic charges, that is by forming conducting paths in the filmor promoting the formation of conducting hexagonal selenium so thatelectrostatic charges leak off rapidly even in the dark andelectrostatic deposition of powder or other finely-divided materialcannot be obtained. Preferably, there should be used amorphous seleniumavailable in pellet form inch to inch size under the name A.R.Q.(ammonia reduced in quartz from selenium oxide) as manufactured, forthis grade of selenium is essentially pure, containing less than abouttwenty parts per million of impurities. If purified, other grades ofselenium, i.e. D.D.Q. (double distilled in quartz) and C.C.R.(commercial grade) as manufactured can likewise be employed in theprocess disclosed herein. To purify these grades of selenium,

Table 2 Initial Dark- Sensitivity Example Film Composition ChargingPotential, Decay Polarity volts Rate,

volts/min. 400 m 500 m 7 Selenium Se+As1S|Interiaee 2 5g 400 1. 6 46 7.l it a a a 2 420 0.554 '11 R an 320 a 10 a 3 3 it i3 42 4 9 280 18.4 258.2

A conductive base plate is usually required for xerothey are first freedof copper, iron, lead, and bismuth by graphic plates and metal forms themost suitable material. However, a high conductivity is not required andalmost any structurally satisfactory material which is more conductivethan the photoconductive layer can be used. Materials having electricalresistivities less than about 10 ohms-cm. are generally satisfactory forthe base plates of this invention although it is more desirable to usematerials of less than about 10 ohms-cm. Any gross surfaceirregularities, i.e. burns, tool marks, are removed from the base plateby grinding or polishing, although it is unnecessary to polish the plateuntil it has a mirror-like surface. The plate surface is cleaned beforecoating in order to remove grease, dirt, and other impurities whichmight prevent firm adherence of the coating to the base plate. This isreadily accomplished by washing the plate with any suitable alkalicleaner or with a hydrocarbon solvent such as benzene, followed byrinsing and drying. Suitable base plate materials are aluminum, glasshaving a conductive coating thereon as of tin oxide or aluminum,stainless steel, nickel, chromium, zinc, etc.

Conductive plastic, conductively coated paper, or other web or film-likemember may be used as the conductive supporting surface as desired. Itis to be understood that the backing member selected for this plate maybe in the form of a flat plate or may equally be in the formdistillation. The selenium is next heated to about 250 C., slightlyabove its melting point, and, while molten, is then dropped through ashot tower (or in. the laboratory by means of an eye dropper) into waterto form pellets. The pellets are subsequently treated with petroleumether to remove water and allowed to air dry. If desired, the purifiedselenium can be remelted and cast in boats to form sticks. It can alsobe reduced in size by grinding or micropulverizing to facilitatemelting.

The thickness of the photoconductor and arsenic trisulfide layers is notat all critical. In general, the arsenic trisulfide interlayer may varyfrom about micron to about 3 microns while the photoconductiveinsulating material may vary from about 10 microns to about 200 microns.

What is claimed is:

1. A xerographic plate comprising a conductive backing having thereon alayer of arsenic trisulfide from about A to about 3 microns thick and alayer of a photoconductive insulating material from about l0 to about200 microns thick on said arsenic trisulfide.

2. A xerographic plate comprising a support member, a layer ofelectrical conductive material, a layer of arsenic trisulfide from aboutA to about 3 microns thick on said conductive material and a layer of aphotoconductive insulating material from about 10 to about 200 micronsthick on said arsenictrisulfide.

3. A xerographic plate comprising a conductive support mcmben alaycnofarsenic tri'sulfide from about toabout3=micronsthiok and a-layerof vitreous selenium-t 5 from about 10 to about 200 microns thick.

A phiq plata. composing a, conductive. sup,- port member, a laye; ofarsenic t isulfidc fnomabout fl to about 3 microns thick, a layer ofvitneous; scl'enium; on said arsenic trisulfidc and a protectiyeinsulatingcoat= l0; ing on said selcnium fromvabout 10 to about 200microns thick.

References Cited in the file of this patent UNITED STATES. PATENTS iWeimer .Aug..24,.1954' Grandadam Oct 19, 1954 Paris Aug. 20, 1957Ullrich Aug. 20, 1957 Rome Feb. 25, 1958 UNITET) STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 2301 349 I August 25 1959 Roland M,Schafiert et a1 It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected 'belo Column 2,, line18,, for "trifluoriole" read trisulfide column 3, line 37,, after thenumeral "2" and before the comma insert a closing parenthesis; column 4,line 26, for "mac-second" read ma-second column '7 lines 1O 11 and 12for "on said arsenic trisulfide and a protective insulating coating onsaid selenium from about 10 to about 200 microns thick." read from about10 to about 200 microns thick on said arsenic trisulfide and aprotective insulating coating on said selenium,

Signed and sealed this 30th day of August 1960,

(SEAL) Attest:

ERNEST W. SWIDER ROBERT Co WATSON Attesting Officer Commissioner ofPatents

1. A XEROGRAPHIC PLATE COMPRISING A CONDUCTIVE BACKING HAVING THEREON ALAYER OF ARSENIC TRISULFIDE FROM ABOUT 1/10 TO ABOUT 3 MICRONS THICK ANDA LAYER OF A PHOTO-